Reciprocating engines for motor vehicles, motorcycles, agricultural machines, marine vessels and the like require a crankshaft to extract power by converting reciprocating motion of pistons to rotary motion. Crankshafts are generally categorized into two types: those of the type produced by die forging and those of the type produced by casting. In particular, in cases where high strength and high stiffness are required, the firstly mentioned forged crankshafts, which are superior in those properties, are often employed.
In general, forged crankshafts are produced by using, as a starting material, a billet having a circular or square cross section and having a constant cross-sectional area along the entire length. In a production process of a forged crankshaft, the billet is subjected to the steps of preforming, die forging, trimming and coining in order. Typically, the preforming step includes the steps of roll forming and bending, and the die forging step includes the steps of block forging and finish forging.
FIGS. 1(a)-1(f) are views schematically illustrating a typical conventional process for producing a forged crankshaft. A crankshaft 1 illustrated in FIG. 1(f) is intended to be mounted in a 4-cylinder engine, and is a 4-cylinder 8-counterweight crankshaft. The crankshaft 1 includes: five journals J1 to J5; four crank pins P1 to P4; a front part Fr; a flange Fl; and eight crank arms (hereinafter referred to simply as “arms”) A1 to A8 that connect the journals J1 to J5 and the crank pins P1 to P4 to each other. The eight crank arms A1 to A8 have counterweights (hereinafter referred to simply as “weights”) W1 to W8, respectively. The weights W1 to W8 are integrally formed with the arms A1 to A8, respectively.
In the following paragraphs, when the journals J1 to J5, the crank pins P1 to P4, the arms A1 to A8, and the weights W1 to W8 are each collectively referred to, a reference character “J” is used for the journals, a reference character “P” for the crank pins, a reference character “A” for the arms, and a reference character “W” for the weights. Also, a crank pin P and a pair of arms A (including weights W) which connects with the crank pin P are collectively referred to as a “throw”.
According to the production method shown in FIG. 1, the forged crankshaft 1 is produced in the following manner. First, a billet 2 shown in FIG. 1(a), which has been previously cut to a predetermined length, is heated by a heating furnace (for example, an induction heater or a gas atmosphere furnace) and then is subjected to roll forming. In the roll forming step, the billet 2 is rolled and reduced in cross section by grooved rolls, for example, to distribute its volume in the longitudinal direction, whereby a rolled blank 3, which is an intermediate material, is formed (see FIG. 1(b)). In the bending step, the rolled blank 3 obtained by roll forming is partially pressed in a direction perpendicular to the longitudinal direction to distribute its volume, whereby a bent blank 4, which is a secondary intermediate material, is formed (see FIG. 1(c)).
Then, in the block forging step, the bent blank 4 obtained by bending is press forged with a pair of upper and lower dies, whereby a forged blank 5 having a general shape of a crankshaft (end product) is formed (see FIG. 1(d)). Then, in the finish forging step, the block forged blank 5 obtained by block forging is further press forged with a pair of upper and lower dies, whereby a forged blank 6 having a shape in agreement with the shape of the crankshaft (end product) is formed (see FIG. 1(e)). In the block forging and the finish forging, excess material flows out from between the mutually opposed parting surfaces of the dies, thereby forming flash. Thus, the block forged blank 5 and the finish forged blank 6 have large flash (5a, 6a) around the shape of the crankshaft.
In the trimming step, the finish forged blank 6 with the flash 6a, obtained by finish forging, is held by dies from above and below, and the flash 6a is removed by a cutting die. In this manner, the forged crankshaft 1 is obtained as shown in FIG. 1(f). In the coining step, principal parts of the forged crankshaft 1, from which the flash has been removed, are slightly pressed with dies from above and below and corrected to the size and shape of the end product. In this regard, the principal parts of the forged crankshaft 1 are, e.g., shaft parts such as the journals J, the crank pins P, the front part Fr and the flange Fl, and in some cases the arms A and the weights W. In this manner, the forged crankshaft 1 is produced.
The production process shown in FIGS. 1(a) to 1(f) is applicable not only for producing a 4-cylinder 8-counterweight crankshaft as illustrated in FIG. 1(f) but also for producing various other types of crankshafts. For example, the production process is applicable for producing a 4-cylinder 4-counterweight crankshaft. In a 4-cylinder 4-counterweight crankshaft, some of the eight arms A have weights W. For example, among the eight arms A, the leading first arm A1, the trailing eighth arm A8, and the two central arms (the fourth arm A4 and the fifth arm A5) have weights W. Also, the same production process can be applied for producing crankshafts that are to be mounted in a 3-cylinder engine, an inline 6-cylinder engine, a V-type 6-cylinder engine, an 8-cylinder engine, and the like. It is noted that, when adjustment of the placement angle of the crank pins is necessary, a twisting step is added after the trimming step.
In recent years, there has been a need for weight reduction of reciprocating engines, particularly those for motor vehicles, in order to improve the fuel economy. Accordingly, there is also an ever-increasing demand for weight reduction of crankshafts, which are a principal component of a reciprocating engine. Conventional techniques intended for weight reduction of a forged crankshaft include the following.
Patent Literatures 1 and 2 each disclose an arm having a hollow portion in the journal-side surface of the arm, and disclose a method for producing a crankshaft having the arm. The hollow portion in the arm is positioned on a line connecting the axis of the journal and the axis of the crank pin (the line hereinafter being referred to as an “arm centerline”), and the hollow portion is depressed greatly and deeply toward the crank pin. The arm is reduced in weight by an amount corresponding to the volume of the hollow portion. The weight reduction of the arm leads to weight reduction of the counterweight, which forms a pair with the arm, and this in turn leads to weight reduction of the forged crankshaft as a whole. Furthermore, each of the arms disclosed in Patent Literatures 1 and 2 has sufficient stiffness (torsional rigidity and flexural rigidity) because the side portions near the crank pin, between which the arm centerline is interposed, have a large thickness.
By providing a recess in the journal-side surface of the arm while ensuring a large thickness at the side portions of the arm as described above, it is possible to achieve weight reduction in combination with sufficient stiffness.
However, forged crankshafts having such a unique shape are difficult to produce using conventional production methods. The reason is that, when the recess in the surface of the arm is to be formed in the die forging step, a situation will occur in which the draft of the die becomes a reverse draft at the site of the recess and therefore the formed forged blank cannot be removed from the die.
To address such a situation, the production methods disclosed in Patent Literatures 1 and 2 are configured as follows: in the die forging step, the arm is shaped to be small with no recess formed in the surface of the arm, and after the trimming step, a punch is pressed into the surface of the arm so that the mark made by the punch forms the recess.
In the meantime, by the production process illustrated in FIGS. 1(a) to 1(f), unnecessary large flash, which is not a part of an end product, is generated, thereby leading to yield reduction. Thus, in the technology for manufacture of forged crankshafts, improving the yield by suppressing flash has been an object. Conventional techniques intended to attain this object include the followings.
For example, Patent Literature 3 discloses a technique for producing a crankshaft comprising shaped journals and crank pins, and roughly shaped arms. According to the technique, the used as a blank is a stepped round bar having reduced diameter regions at portions to be formed into journals and crank pins of a crankshaft, and portions to be formed into a pair of journals between which a crank pin intervenes are held with dies.
In this state, the dies are axially moved closer to each other to compressively deform the round bar blank, and a punch is applied to the portion to be formed into a crank pin in a direction perpendicular to the axial direction, whereby the portion to be formed into a crank pin is pressed into an eccentric position. This operation for pressing the portion to be formed into a crank pin into an eccentric position is repeated in succession for all crank throws. In this way, the journals and the crank pins are shaped, and the arms are roughly shaped.
Also, Patent Literature 4 discloses a technique for producing a crankshaft comprising shaped journals and crank pins, and roughly shaped arms. According to the technique, the used as a blank is a simple round bar. One of the two ends of the round bar is held with a stationary die, and the other end thereof is held with a movable die. Also, portions to be formed into journals are held with journal dies, and portions to be formed into crank pins are held with crank pin dies.
In this state, the movable die, the journal dies and the crank pin dies are axially moved toward the stationary die to compressively deform the round bar blank. At the same time, the crank pin dies are moved in an eccentric direction perpendicular to the axial direction to press the portions to be formed into the crank pins into eccentric positions. In this way, the journals and the crank pins are shaped, and the arms are roughly shaped.
With both the techniques disclosed in Patent Literatures 3 and 4, no flash is generated, and therefore, a significant improvement in yield can be expected.